Research programs often make significant advancements when appropriate new analytical and experimental procedures are developed. We have developed and X-ray fluorescence technique for measuring elements in biological tissues that will provide advanced analytical capability which previously has not been possible. To take full advantage of this achievement, we are requesting funds for the purchase of an X-Ray Fluorescence (XRF) Spectrometer system. In the basic operation of an XRF spectroscopy system, a photon bombardment of the sample produces X-rays characteristic for each element which pass through a beryllium window of the sample, strike a detector and are analyzed by a multichannel analyzer. This instrument detects all elements with mol wt greater than 19. The principle investigator developed new techniques utilizing a KEVEX 0700 XRF spectrometer for detecting and quantitating several elements in concentrations greater than 10 umolar, either separately or simultaneously, in various biological tissues. With these technical advances in sample preparation, XRF spectroscopy will simplify many analytical procedures which are now quite tedious and expensive in terms of time and resources. The primary need for the XRF system in our research laboratories is to gain 100% access to the equipment. This instrument will be used in NIH projects related to the development of new contrast enhancement agents for MRI, CT and ultrasonography and for developing new non-invasive MRI diagnostic and physiologic techniques. Current access to the one XRF system in the San Diego region (North Island Naval Base) is limited, inconvenient, terribly inefficient and severely restrictive to the progress of our NIH research programs. When accessible, the XRF system is still only available for a small fraction of the time needed to take full advantage of this technology for our NIH programs. Because of the instrument's versatility and the technical advances we are making in sample preparation, this XRF system will be useful for many years. It will remain an ideal core instrument, suitable for today's important NIH-sponsored projects while flexible enough to be relevant for future projects --- an invaluable tool for reaching today's and tomorrow's research goals.